In recent years, there is a tendency that weight reduction of automotive members is emphasized from the viewpoint of energy saving and safety and durability thereof are also additionally emphasized, and thus higher strengthening is rapidly progressing than ever before. As an example of this tendency, a high-strength steel sheet is adapted to be applied not only to outer panels of an automobile but also to structural members.
The steel sheet to be applied to such structural members also requires workability such as hole expandability in addition to press formability. For this reason, a high-strength hot-rolled steel sheet having excellent workability in a burring work, a stretch flanging work or the like has been developed (for example, see Patent Literatures 1 and 2).
However, with the higher strength of the hot-rolled steel sheet, there is a problem that peeling or burr-like defects occur in an end face of a hole formed by a punching work of the steel sheet. These defects significantly impair a design nature in the end face of the product and also have a risk of affecting fatigue strength or the like as a stress concentration portion.
With respect to the above problems, a hot-rolled steel sheet has been proposed in which an area ratio of a second hard phase and cementite is restricted and the damage is suppressed in the punched end face (for example, see Patent Literatures 3 and 4). However, even though the formation of the second hard phase and cementite is suppressed, when a clearance of the punching work is set to the most severe condition to the damage of the end face, there are cases where the defects occur in the end face of the hole.
In contrast, a high-strength hot-rolled steel sheet has been developed in which B is added or the adding amount of P is limited so as to suppress a fracture in crystal grain boundaries during working and thus the damage occurrence in the punched end face is suppressed (see Patent Literatures 5 and 6). Furthermore, a high-strength hot-rolled steel sheet has been developed in which the segregation amount of C or C and B is controlled in large-angle crystal grain boundaries of ferrite and thus the damage occurrence in the punched end face can be prevented even when the punching work is carried out under the most severe conditions (see Patent Literatures 7 and 8). However, the steel sheets disclosed in Patent Literatures 5 to 8 include a structure mainly containing a ferrite phase. Accordingly, these steel sheets were difficult to achieve high strength of 850 MPa or higher.